September 20, 2010
‘How many signals are present?’ is such a simple question, and yet, it is a fundamental question to an elucidator. Mistaken a signal or overlook one and the elucidator can run the risk of wasting time and effort.
The 13C [1H] NMR spectrum below shows 3 discernible signals that are attributed to the solvent CDCl3. In addition, one can begin to speculate on weaker signals; there might a fourth signal at ~77.2 ppm (most likely due to residual CHCl3), possibly a fifth at ~77.5 ppm, perhaps a sixth one at ~77.3 ppm and maybe more.
The next step is to examine additional data and verify whether the ‘weak’ signals are real or not. This can include:
1. comparing the weak signals to other structural signals,
2. applying deconvolution/peak fitting to this region,
3. checking 2D NMR data,
4. acquiring data in a different solvent,
5. modifying the acquisition parameters to exclude the solvent or increase S/N, etc.